Medical devices can be sterilized in various ways, such as by gas and radiation. Gas sterilization may not be suitable for various reasons. For example, there may be a need to sterilize tight or closed confines of the medical device which cannot be reached by the sterilizing gas with sufficient reliability. Also, the use of gas may result in undesirable chemical reactions with drugs or materials of the medical device. Packaging required for some medical devices may also limit the effectiveness of gas sterilization. Accordingly, it may be more desirable or necessary to sterilize some medical devices using radiation, such as electron beam (E-beam), gamma radiation, ion beam, and x-ray.
Some forms of radiation have been used to sterilize conventional metal stents. A stents is a type of endoprosthesis that is implanted in an anatomical lumen. An endoprosthesis is an artificial device that is placed inside a human or animal body. An anatomical lumen is a cavity of a tubular organ such as a blood vessel. Stents are generally cylindrically shaped devices, which function to hold open and sometimes expand a segment of a blood vessel or other anatomical lumen such as urinary tracts and bile ducts. Stents are often used in the treatment of atherosclerotic stenosis in blood vessels. In such treatments, stents reinforce body vessels and prevent restenosis following angioplasty in the vascular system.
Radiation used for sterilization can have a negative effect on the chemical, mechanical and other properties on the item being sterilized if radiation exposure is not controlled. There is often a greater need to minimize such negative effects for stents as compared to other types of medical devices. This is because of the unique structural and functional requirements imposed on stents.
Stents are relatively small, as they are often required to be passed through tight confines of anatomical lumens. A stent must often have great longitudinal flexibility to allow it to pass through tortuous curves of anatomical lumens. Stents typically comprise a fine network of struts which form a tubular scaffold. The tubular scaffold must often be capable of being crimped onto a delivery device, such as a balloon, to reduce its size to allow passage through anatomical lumens, and then forcibly expanded by the balloon to an enlarged, deployed state at the desired location within the body. For some stents, the tubular scaffold must be capable of self-expanding from its crimped state at the desired location within the body. After implantation and deployment, the tubular scaffold must have sufficient strength to support surrounding anatomical structures upon implantation. Thus it will be appreciated that stents present unique challenges in controlling radiation exposure to minimize negative effects on the stent while at the same time reducing the bioburden of the stent to an acceptable sterility assurance level (SAL).
Also, polymers are often more susceptible to negative effects of radiation sterilization than metals. Some polymer stents are designed to biodegrade after implantation and it may be desirable that the stent degrade or dissolve at a particular rate. Thus, the advent of polymer stent can present even greater challenges. An example of a polymer stent and method of manufacture is provided in U.S. application Ser. No. 12/558,105, filed Sep. 11, 2009 (Publication No. 2011/0066222), the entirety of which is incorporated herein by reference.
Additionally, an active or bioactive drug may be contained in a coating on a stent or within polymer struts of a stent. The drug may be required to elute at a particular rate upon implantation. Certain drugs may be sensitive to radiation or heat induced by radiation exposure. Thus, the need to prevent degradation of the drug can present further challenges.
Furthermore, the cost of sterilization is lowered when the number of stents sterilized over a period of time (i.e., sterilization capacity or throughput) is increased. Thus, a further challenge is to develop ways to increase efficiency of radiation sterilization procedures while at the same time maintaining acceptable sterility assurance levels and reducing negative effects of radiation sterilization on stents.
Accordingly, there is a continuing need for methods and apparatuses for radiation sterilization of medical devices which allow for greater control of radiation exposure in order maintain acceptable sterility assurance levels, minimize negative effects on the medical device, and increase sterilization throughput.